The present invention relates to telecommunication systems in which asynchronous transfer mode (ATM) is used for transporting voice-type data as well as other types of data, for example, video and control data. More particularly, the present invention relates to a telecommunication system in which ATM is used for transporting low bit rate, circuit emulation data (i.e., synchronous data) from one or more circuit emulation connections (i.e., circuit emulation data sources).
ATM is a standard protocol that is commonly used for transmitting asynchronous telecommunication data within a telecommunication system for one or more applications. ATM is based on the transmission of data in fixed size data packets known as ATM cells. The protocol for each ATM cell is the same, wherein, each ATM cell contains a 48 octet payload and a 5 octet header. In general, ATM is well known in the art.
The telecommunication data associated with each application is initially in a data transfer format that is application specific. If ATM is to be used for transporting the data, the application specific data format is adapted so that it is compatible with the ATM protocol. This is accomplished by an ATM adaptation layer (AAL) 101, as illustrated in FIG. 1. Referring now to FIG. 1, the application layer 102 represents telecommunication data arriving from a specific telecommunication data application. The task of the AAL 101, as mentioned, is to reformat the data so the data is compatible with the ATM protocol. Once reformatted, the ATM layer 103 can transport the data to a desired receiving unit.
One of the more common AALs is AAL1. AAL1 is typically used to packetize synchronous data (i.e., circuit emulation data) into standardized data packets, which can, in turn, be structured or unstructured data. Structured data is organized into a sequence of data blocks, wherein the boundary for each data block is defined by a structured data pointer (SDP). The SDP is specifically used for alignment (i.e., recovery) of the data at a receiving unit. Unstructured data refers to raw data that includes no framing information.
AAL1 is divided into two basic functional sublayers, as illustrated in FIG. 1: a segmentation and reassembly (SAR) sublayer 104 and a convergence sublayer 105. The SAR sublayer 104 packetizes the incoming data into data blocks that are 47 bytes in length. The SAR sublayer 104 then adds a 1 byte sequence number and a 1 bit data type identifier (to identify the incoming data as either structured or unstructured). For example, if the data type identifier bit is set, the first byte in the block will contain a SDP. The convergence sublayer supports data packetization, clock recovery, cell delay variation compensation and forward error correction.
There are a number of inherent problems associated with AAL1. Foremost is that the time delay required by AAL1 to prepare a 47 byte data block is excessively long. For example, a typical service rate (i.e., the incoming data rate) for circuit emulation data is 64 kbits per second. The corresponding time delay for AAL1 would be approximately 6 milliseconds (i.e., 47 bytes/8 kbytes per second). Moreover, the transportation of data from a sending unit to a receiving unit typically involves several ATM transitions; thus, the already excessive delay is compounded with each ATM transition. In addition, when dealing with low bit rate data, there is often an insufficient amount of data to completely fill each ATM cell. Pursuant to the ATM cell protocol, the AAL1 may have to fill the remaining portion of each ATM cell with padding bits. This, in turn, results in poor bandwidth utilization. Since many applications, such as voice data, are highly sensitive to data transportation delays, and because bandwidth is very expensive, there is a real need to design a more efficient way to transport low bit rate, circuit emulation data using ATM.